1. Field of the Invention
The present invention is related to optical devices; more particularly, the invention relates to non-mechanical optical switches.
2. Description of Related Art
Optical switch is a device for directing optical signals along selected fibers of an optical network, in which light signals are transmitted along optical fibers to transfer information from one location to another. The desirable optical switch performance include: high speed switching, bi-direction operation, low optical insertion loss, long operation lifetime, small size, and low cost. Optical switch is a key component in today""s optical network, analogous to the electrical switches in electrical networks. However, it has not been widely adopted because its lack of reliability and its high cost associated with its fabrication difficulty.
In an optical switch, light signal must be accurately entered into an optical fiber, or much of the signal strength will be lost. The alignment requirements of modem single mode optical fibers are particularly stringent, as their core diameters are typically as small as 2 to 10 micrometers and their acceptance angle is fairly narrow. Additional insertion losses reduce the amplitude of the optical signal. Therefore, optical switches which accept light from an input optical fiber, and which selectively couple that light to any of a plurality of output optical fibers must transfer that light with precise alignment and within the small acceptance angle for light to efficiently enter the fiber. Most prior art optical switches are based on mechanical movement to switch light beams, consequently have drawbacks of slow and less reliable. It is greatly desirable to have optical switches that direct light beams without moving parts, a feature generally associated with high reliability and high speed.
Many types of alternative none-mechanical optical switches have been developed for commercial applications, such as thermal heating, electro-optic phase retardation, and magneto-optic polarization rotator. These devices use various materials and configurations. Thermal heating based switches typically rely on long interaction thin film waveguide construction (for example U.S. Pat. No. 5,892,863). This type of switch has a deficiency of large insertion loss due to fiber to thin film waveguide coupling. On the other hand, micro-optic assembly generally provides low optical loss. Presently, the dominant low loss electro-optic switches are based on organic liquid crystal materials. This type of device, consequently, has undesirable properties of slow operation at low temperature and electrodes in the light path (for example U.S. Pat. No. 4,917,452).
Among non-mechanical optical devices, oxide crystal materials such as magneto-optic and electro-optic based micro-optic devices are particularly attractive. Inorganic materials are generally preferred than organic materials in optical network devices, due to their excellent stability. Optical switches based on magneto-optic crystals have been described in several patents, as referenced in this disclosure. However, all known magneto-optic switch designs are limited to transmit light only in one direction. This deficiency hampers their applications in today""s optical networks, which are often bi-directional. Moreover, due to their complex configurations, conventional optical switches also suffer from high insertion loss and high cost. Further, these switches often comprise many elements and require extremely stringent alignment that is unsuitable for manufacture. Therefore, magneto-optic based optical switches have not been widely used in optical communications.
An early concept of a magneto-optic crystal based optical switch for telecommunication use was disclosed by Jin, U.S. Pat. No. 5,627,924. The switch is a modification of optical circulator thus has a drawback of non-reciprocal operation. In Jin""s switch, the optical beams from the two ports on the same side propagate desirably parallel but with a relative large spatial location shift. This design requires three individual imaging lenses, or three fiber collimators. A fiber collimator is a component consists a collimating lens packaged together with light guiding fibers. Because of the large beam separations between the two adjacent lenses, the design requires large and long crystals to deflect the beams. As a result, the optical device typically has large loss, excessively large size, and is expensive to produce.
Recent version as described by Shirasaki U.S. Pat. No. 5,982,539 represents some improvement by using dual fiber sharing a single imaging lens to reduce the optical device size. However, Shirasaki switch is a non-reciprocal device, which is unsuitable for modern bi-directional communication applications. Another disadvantage of Shirasaki switch is that the beam propagations are no long parallel rather with an angle. Consequently the switch demands precise fabrication of polarization prisms and matching birefringent wedges. This switch also requires delicacy for maintaining accurate alignment of each optical path, in which the angular and the spatial positions are closely interrelated. Therefore, manufacturing of Shirasaki switch is difficult.
Recent switches as described by Bergman, U.S. Pat. No. 5,923,472 and U.S. Pat. No. 6,173,092B1 and Robinson, U.S. Pat. No. 5,933,269 all utilize mirrors to reflect the beam back to another port on the same side. With a long and fold beam propagation and an unsymmetrical geometry, the devices become less tolerance to both extremely small angle and position misalignment. This type of switch is therefore often very difficult and costly to make. Moreover, the design is a non-reciprocal device.
For the above reasons, what is needed is a system and method for providing non-mechanical optical switches that is bi-directional in light transmission and amenable to volume production. It would be particularly desirable to provide optical switches having low optical insertion loss and high speed switching that is also reliable, but which use less components of small size and require reduced alignment steps with large assembly tolerance to facilitate low cost manufacture.
It is an object of the present invention to provide a Faraday rotator assembly in which polarization rotation can be electrically controlled between 0xc2x0 and 90xc2x0, forming a base for bi-directional devices. It is another object of the present invention to provide a compact and economical non-mechanical optical switch that can be efficiently coupled to optical fibers. The invention consists of optical switches having at least three ports for optical fibers. The inventive switches use at least one single lens to coupling two fibers for compactness. The invention further consists of a light-bending device, situated to compensate for the angle between the two light beams that share the same lens, advantageously increasing alignment tolerance.